The invention relates to a fuel cell battery with an integrated heat exchanger. It also relates to a plant with the fuel cell battery in accordance with the invention.
A fuel cell battery having a cylindrical cell stack is known from EP-A 1 037 296 in which afterburning is carried out at the periphery of the stack. The afterburning is carried out using educts which have not been converted in the current supplying electrochemical reactions in the cells. The educts are a gaseous fuel (in brief, fuel gas), on the one hand, i.e. a mixture which includes reducing components, in particular hydrogen and carbon monoxide, and a gas with oxidizing modules, on the other hand, in particular a gaseous oxygen carrier, for example in the form of heated environmental air. Each cell of the stack has at least one inlet point for the oxygen carrier. The afterburning is provided within a ring-shaped region around the cell stack. The inlet points are either all connected in a communicating manner as an entity, or are connected in a communicating manner group-wise, via at least one air space which extends axially along the cell stack and which is in direct contact with this. Each air space is separated by a wall from a chamber for the afterburning which likewise forms a space communicating axially along the cell stack. Each fuel cell includes two parts, namely a so-called PEN element (called PEN in brief and a disc-shaped inter-connector. The PEN element, which consists of at least three layers, namely P (cathode=positive electrode), E (electrolyte) and N (anode), is an electrochemically active element with which the electrochemical reactions can be carried out; it has the form of a thin, circular disc which consists, for example, of a layer-shaped solid electrolyte and two electrodes, P and N respectively, applied by coating. The inter-connector separates a space for the oxygen carrier from a space for the fuel gas. It has an architecture with a relief-like profile by means of which a flow of the fuel gas from a central inlet point is made possible along the PEN to the periphery. On the other hand, a transport of the oxygen carrier is guided by the special architecture and leads from the air chamber or from the air chambers to the center and from there along the PEN back to the periphery. Discretely arranged openings for the inlet or the outlet of the gases are disposed at the periphery.
A jacket, which envelopes the cell stack in the known fuel cell battery, is made as a heat insulation system. Its heat insulating function plays the role of an external recuperator. Instead of the oxygen carrier required in the cells for the electrochemical processes first being pre-heated in a separate external recuperator, the initially cold oxygen carrier is used as a heat sink in that the heat flowing away from the cell stack is partly absorbed in the jacket by the oxygen carrier and is returned to the reaction point or zone.
The known jacket is made with multiple layers; it has a passage system for the flow of the oxygen carrier. A first hollow space, in which a distribution and a heating of the oxygen carrier or a cooling of the jacket takes place, is disposed between an outer wall, which forms a first layer of the jacket, and the inner parts of the jacket. A further heating of the oxygen carrier results in the passage system which adjoins the first hollow space. Instead of, or in addition to, the passages, porous, gas permeable parts can also be installed in the jacket which form a so-called dynamic heat insulation: the oxygen carrier, which flows through the pores of the heat insulation in the radial direction, absorbs heat which is mainly emitted from the cell stack by heat radiation, and is absorbed by the material of the heat insulation. The absorbed heat is transported back into the cell stack by the oxygen carrier.
The afterburning chambers are made as axially directed collecting passages through which the exhaust gas can be led away, in particular sucked off. When moving from the jacket into the cell stack, the oxygen carrier is heated up further at the outer walls of the afterburning chambers; heat is accordingly given off by the exhaust gas flowing axially in the chambers, the heat corresponding to the heat arising during the afterburning and to a part of the heat released by the electrochemical reactions.
As a rule, the largest possible electrical power should be achieved with a system in which fuel cells are used for an energy conversion. In this connection, the electrochemical reactions are carried out in a steady state of the fuel cell system and under conditions for which an optimal utilization of the system results with respect to the efficiency of the reactions and a temperature-dependent ageing of the PEN elements.
In one design of the fuel cell system, the calculation of an energy balance has to be carried out with the following amounts of heat being involved in the calculation for a steady state: the amounts of heat produced in the reactions and during the afterburning; the heat loss, i.e. the amount of heat which flows away through the jacket to the environment; and the excess amount of heat which is led away out of the system with the exhaust gas. Various parameters play a role in this connection, including: the temperatures in the cells, the temperatures in the chambers for the afterburning, the air ratio λ, or another corresponding parameter. (λ is the ratio between the mass flows of the air supplied and the stoichiometrically required amount of air, with air being the oxygen carrier.) The fuel cell system can be designed such that the reaction temperature is ideal and is largely equally high in each cell. This design is carried out with respect to a full load, i.e. with respect to the maximum electrical power which can be achieved under optimum conditions.
The fuel cell system can be used in a plant which is part of a building infrastructure, with the energy converted by the fuel cells being used in the form of thermal energy (for example for heating purposes) and of electrical energy. Since the energy supply of a building must follow variable requirements, it is also necessary for the fuel cell system to be able to be operated at part load. At part load, the supply flows for the educts are reduced; the reaction temperature must, however, still be maintained—at a value, for example, of 900° C. Since the heat loss at part load is higher than at full load (because the dynamic heat insulation performance of the jacket becomes smaller due to a reduced air supply), there is no linear relationship between the amount of energy converted and the mass flows of the educts. The operation of the fuel cell system can be regulated in line with requirements and according to the non-linearities with a suitably designed control system.
There is a further problem in this connection, and indeed with respect to the design of the system, in which there is the aim of not allowing any axial temperature gradients to arise in the cell stack. It has been found that, at part load, a temperature gradient cannot be prevented along the cell stack if this temperature gradient is zero at full load due to the design: the exhaust gas flowing away axially in the afterburning chambers is subjected to a reduction in its temperature due to the heat transfer to the jacket; because the heat reserve from the cell stack cannot fully compensate the heat transfer to the jacket at part load. This results in the temperature gradients of the cell stack, and indeed with a decreasing temperature in the flow direction of the exhaust gas. It is therefore no longer possible to carry out the electrochemical reactions in all fuel cells at the optimum temperature of full load operation.